Method for electrolytic coating of materials with aluminum, magnesium or aluminum and magnesium alloys

ABSTRACT

The invention concerns a method for electrolytic coating of materials with aluminum, magnesium or aluminum and magnesium alloys. Said method is characterized in that the material is pretreated by being immersed in an electrolytic solution, where it is anodized, the electrolytic coating being performed immediately after in the same electrolytic solution.

The present invention relates to a method for electrolytic coating ofmaterials with aluminum, magnesium or aluminum and magnesium alloys, inwhich method the material is immersed in an electrolyte forpretreatment, being connected as anode therein, and electrolytic coatingis performed in the same electrolyte immediately thereafter. The methodaccording to the invention improves the quality of the depositedaluminum, magnesium or aluminum/magnesium coatings.

Deposition of aluminum, magnesium or aluminum/magnesium alloys onmaterials consisting of base metals is a convenient way of protectingsuch materials from corrosion. At the same time, they are provided witha decorative coating. To this end, the protective metal layer ispredominantly deposited on the material by means of electroplating.Advantageously, the aluminum, magnesium or aluminum/magnesium layer iscoated on the material with no coating of metallic intermediate layersbetween said metal layer and said material. If intermediate layers havebeen coated between the material and the surface layer of aluminum,magnesium or aluminum/magnesium alloy, there is a risk of contactcorrosion due to the coated intermediate layer. In addition, thermalproblems may arise due to the different expansion coefficients of thesurface layer and intermediate layer.

Electrolytes found useful in the prior art include fused-saltelectrolytes, such as electrolytes containing aluminum halides oraluminum alkyl complexes. A common feature in all of these electrolyticsystems is that the material has to be cleaned prior to coating thesurface thereof. Thus, particularly with materials consisting of basemetals forming an oxide layer, there is a problem in that such oxidelayers must be completely removed prior to coating. If the surface ofsuch materials has not been completely cleaned, impurities or residuesof the oxide layer of the metal constituting the material, which adhereto the surface, result in impaired adhesion of a metal layersubsequently coated by electrolysis. Furthermore, it is possible that nometal layer at all is coated on those spots where impurities are presenton the surface, because said impurities normally are electricallynon-conductive, thereby preventing electrolytic deposition at that spot.This inevitably gives rise to corrosion problems of the finished coatedmaterial at those spots where coating of the metal layer is incomplete.

DE-C3-22 60 191 describes a method of preparing materials made ofelectroconductive materials. In this method, the last process step,which serves to shape the materials, and in which a new bare surface isformed on the material, is performed in a suitable inert gas or inertfluid medium, with exclusion of atmospheric oxygen and moisture. Thismethod turned out to be disadvantageous in that, particularly when usingan inert fluid medium that covers the surface of the material andtherefore might enter the coating electrolyte, the electrolyte issubsequently contaminated or hydrolyzed by same. When using inert gasmedia, one problem arising in large-scale industrial applications isthat an inert gas atmosphere absolutely free of oxygen cannot beaccomplished in practice. Traces of oxygen present in the inert gasatmosphere immediately oxidize the bare metal surface of the material,thus giving rise to the above-described loss in quality of the metallayer subsequently coated by means of electroplating. If, as describedin DE-C3-22 60 191, the bare surface is achieved by means of amechanical procedure, such as milling, cutting, sawing or drilling, orby means of massive deformation of the material using e.g. rolling orwire drawing, extrusion or other procedures, such procedures will giverise to an increased production tolerance of the finished material. As aresult, materials produced according to the above method are notsuitable for applications where highly constant quality andmanufacturing are required.

DE-AS-12 12 213 describes the pretreatment of a material in a protectivegas atmosphere. Alternatively, the oxide layer on the surface of thematerial can be removed by connecting the material as anode prior todeposition of the aluminum layer in the electrolyte which is producedfrom sodium fluoride and triethylaluminum. Thereafter, the current isreversed, and aluminum is deposited on the material. Disadvantageously,it has been found that the electrolyte can only be used in thedeposition of aluminum on materials. Deposition of magnesium oraluminum/magnesium layers is not possible because the presence of halideions in the electrolyte would result in immediate formation of insolublemagnesium halide compounds during anodic polarity, preventing depositionof magnesium or aluminum/magnesium on the material. The magnesiumhalides being formed would immediately stop the current in theelectrolyte by blocking the electrodes.

DE-AS-21 22 610 describes a method for the anodic pretreatment of lightmetals for the electrodeposition of aluminum. Cleaning of the componentsis effected by treating the light metal materials in a fusedelectrolyte, thereby subjecting the materials to anodic load. The lightmetal materials cleaned in this way, being wetted with electrolyte, i.e.still loaded with fused electrolyte, are immersed in an aluminizingcell. During this operation, the possibility of oxygen still reachingthe pretreated material, re-oxidizing it on the surface thereof, cannotbe excluded. Furthermore, the aluminizing electrolyte is contaminated bythe surface treatment electrolyte, which is a fused electrolyte. Only inthose cases where the material consists of beryllium or aluminum, thefused electrolyte used in surface treatment by anodic oxidation of thematerial can also be used in the electrodeposition of aluminum on theberyllium or aluminum material. The fused electrolyte described inDE-AS-21 22 610 is only suitable for pretreatment of beryllium oraluminum materials in order to effect subsequent coating thereof withaluminum in the same fused electrolyte. The fused electrolyte is notsuitable for electrodeposition of aluminum, magnesium oraluminum/magnesium layers on other materials.

DE-A1-198 55 666 describes an electrolyte suitable for the deposition ofaluminum/magnesium alloy layers. The organoaluminum electrolytedisclosed therein contains K[AlEt₄] or Na[Et₃Al—H—AlEt₃], Na[AlEt₄], aswell as trialkylaluminum. The electrolyte can be present in the form ofa toluene solution. Electrolytic deposition of aluminum/magnesium alloylayers from the electrolyte described therein is effected using asoluble aluminum anode and a likewise soluble magnesium anode, or usingan anode made of aluminum/magnesium alloy. In the described method, theelectrolyte composition is adjusted by pre-electrolysis in such a waythat the deposited layer has the desired aluminum/magnesium ratio.Alternatively, Mg[AlEt₄]₂ can also be added to the electrolyte. Thus,the teaching of DE-A1-198 55 666 is that the ratio of aluminum andmagnesium in the deposited aluminum/magnesium layer strongly depends onthe concentration ratio of magnesium and aluminum in the electrolyte. Asin all prior art methods, great care must be taken in the pretreatmentof the materials to be coated, because impurities in the surface of thematerials caused by oxidation or other influences result in reducedquality of the metal layer deposited by electroplating.

The technical object of the present invention is to provide a method,which method allows coating of aluminum, magnesium or aluminum/magnesiumlayers on materials, the quality of the metal coating being increased byan improved pretreatment of the material. More specifically, a method isto be provided wherein the materials to be coated are made free ofadhering oxide layers or other impurities in a reliable and economicfashion, and wherein the intention is to prevent the materials frombeing recontaminated or reoxidized after pretreatment of the materials.

The technical object of the present invention is accomplished by meansof a method for the electrolytic coating of materials with aluminum,magnesium, or alloys of aluminum and magnesium, in which method thematerial is immersed in an electrolyte for pretreatment, being connectedas anode therein, and electrolytic coating is performed in the sameelectrolyte immediately thereafter, the electrolytic bath includingorganoaluminum compounds of general formulaM[(R¹)₃Al—(H—Al(R²)₂)_(n)—R³] (1) and Al(R⁴)₃ (II) as electrolyte,wherein n is equal to 0 or 1, M is sodium or potassium, and R¹, R², R³,R⁴ can be the same or different, R¹, R², R³, R⁴ being a C₁-C₄ alkylgroup, and a halogen-free, aprotic solvent being used as solvent for theelectrolyte. The method according to the invention allows pretreatmentof the material in that bath wherein electrolytic coating takes placelater on. Surprisingly, impurities adhering to the non-pretreatedmaterial, as well as oxide layers present on the material, are removed.Surprisingly, the impurities thus introduced into the electrolytic bathdo not impede the deposition of magnesium, aluminum or alloys ofaluminum and magnesium on the material. Insoluble impurities can beremoved continuously from the electrolytic bath, using suitablefiltration systems.

Transfer of the materials from the pretreatment bath into theelectrolytic bath after pretreatment is therefore no longer necessary.The above step, invariably involving the risk of re-contaminating thesurface of the material, can thus be avoided.

In a preferred embodiment, an electrolyte in the form of a mixture ofthe complexes K[AlEt₄], Na[AlEt₄] and AlEt₃ is employed in the methodaccording to the invention. The molar ratio of complexes to AlEt₃ isfrom 1:0.5 to 1:3, with a ratio of 1:2 being preferred.

In a preferred embodiment, 0 to 25 mole-%, preferably 5 to 20 mole-%Na[AlEt₄] is employed, relative to the mixture of the complexes K[AlEt₄]and Na[AlEt₄].

In a preferred fashion, a mixture of 0.8 mol K[AlEt₄], 0.2 molNa[AlEt₄], 2.0 mol AlEt₃ in 3.3 mol toluene can be used as electrolyte.

Alternatively, a mixture of Na[Et₃Al—H—AlEt₃] and Na[AlEt₄] and AlEt₃can be used as electrolyte in the method according to the invention. Themolar ratio of Na[Et₃Al—H—AlEt₃] to Na[AlEt₄] is preferably from 4:1 to1:1, with a ratio of 2:1 being preferred. It is also preferred that themolar ratio of Na[AlEt₄] to AlEt₃ is 1:2.

In another preferred embodiment, a mixture of 1 mol Na[Et₃Al—H—AlEt₃],0.5 mol Na[AlEt₄] and 1 mol AlEt₃ in 3 mol toluene is used aselectrolyte.

The electrolytic coating of materials with magnesium, aluminum oraluminum/magnesium alloys is preferably performed at a temperature offrom 80 to 105° C. Preferred is an electroplating bath temperature of 91to 100° C.

The electrolytic deposition of aluminum, magnesium or aluminum/magnesiumlayers on said materials is carried out using a soluble aluminum anodeand a likewise soluble magnesium anode, or using an anode made of analuminum/magnesium alloy. However, sole use of an aluminum or magnesiumanode is also possible.

In the method of the invention, said anodic connection of the materialfor pretreatment can be maintained for a period of from 1 to 20 minutes,with 5 to 15 minutes being preferred.

The anodic load of the material required for pretreatment is effectedusing a current density of 0.2 to 2 A/dm², preferably 0.5 to 1.5 A/dm².

Any material suitable in electrodeposition can be used as material. In apreferred fashion, the material consists of a metal and/or metal alloyand/or is a metallized electrolyte-resistant material which can bedissolved in the electrolyte by means of anodic connection. Thematerials to be coated are preferably rack goods, bulk materials orcontinuous products such as wire, square-section sheet metal, screws ornuts.

The method according to the invention is remarkable in that impuritiesor oxide layers adhering to the materials are removed in a reliablefashion. Surprisingly, there is no adverse change in the electrolytecomposition that would obstruct high-quality deposition of aluminum,magnesium or aluminum/magnesium layers on such materials. Furthermore,the electrodeposited metal layers are coated on the material in a firmlyadhering and homogeneous fashion, because there is no recontamination ofthe material after cleaning. In addition to the advantages in qualityspecified above, cost-effectiveness in coating machined parts with metallayers is achieved by means of the above-mentioned process steps.

Without intending to be limiting, the inventive method for theelectrolytic coating of materials with magnesium, aluminum or alloys ofaluminum and magnesium will be illustrated with reference to thefollowing examples.

EXAMPLES Example 1

Phase a) A stamped part of an AlMg₃ alloy was first subjected toalkaline scouring for 2 minutes in a solution of 100 g/l NaOH at atemperature of 60° C. After washing in water the part was pickled in 10%nitric acid, subsequently washed in distilled water and dried.

Phase b) The dry part was introduced in a coating cell flooded withargon or nitrogen and, following prewashing in toluene, immediatelyintroduced into the coating electrolyte. A mixture of the complexesK[AlEt₄], Na[AlEt₄] and AlEt₃ dissolved in toluene was used aselectrolyte. A plate of AlMg25 alloy was used as counterelectrode. Theproduct to be coated was first connected as anode and treated for 5minutes at a current density of 1 A/dm² and an electrolyte temperatureof 95° C. Subsequently, the polarity was reversed without removing thepart from the electrolyte, followed by immediate coating for 45 minutesat a current density of 1.5 A/dm². A layer of AlMg alloy about 14 μm inthickness was deposited.

The adherence of the layer was checked using the cross-hatch adhesiontest and heat shock test (1 hour at 220° C. and quenching in coldwater). The deposited layer was found to have excellent adhesion on thebase material. No detaching or blistering was detected.

Comparative Example 1

A part treated as comparative sample was treated and coated as inExample 1, but without previous anodic connection. In the cross-hatchadhesion test, it was possible to peel off the layer in the form of afoil. The layer exhibited blisters in the heat shock test.

Example 2

A magnesium AZ-91 alloy pressure-cast part was corundum-blast (granularsize 0-50 μm) at 2 bars. Thereafter, the part was immediately introducedinto the inert gas atmosphere of the coating cell, prewashed in tolueneand immersed in the electrolytic bath as described in Example 1. Theproduct to be coated was first connected as anode for 10 minutes at acurrent density of 1 A/dm². During this period, a layer of about 2 μmwas removed from the product surface. Thereafter, the polarity wasreversed, and the part was connected as cathode for 1 hour at 1.5 A/dm².An AlMg layer with a content of 23-25% Mg and a layer thickness of about18 μm was deposited.

Subsequent adherence tests showed no layer detachment both in thecross-hatch adhesion test and heat shock test.

1. A method for electrolytic coating of a material with aluminum,magnesium or alloys of aluminum and magnesium, said method comprisingimmersing the material in an electrolytic bath comprising electrolytefor pretreatment, wherein said material is connected as anode therein,and performing electrolytic coating in the same electrolyte immediatelythereafter, the electrolytic bath comprising organoaluminum compounds ofgeneral formulas (I) and (II)M[(R¹)₃Al—(H—Al(R²)₂)_(n)—R³]  (I)Al(R⁴)₃  (II) as electrolyte, wherein n is equal to 0 or 1, M is sodiumor potassium, and R¹, R², R³, R⁴ can be the same or different, R¹, R²,R³, R⁴ being a C₁-C₄ alkyl group, and a halogen-free, aprotic solventbeing used as solvent for the electrolyte.
 2. The method according toclaim 1, wherein a mixture of the complexes K[AlEt₄], Na[AlEt₄] andAlEt₃ is employed as electrolyte.
 3. The method according to claim 2,wherein a molar ratio of said complexes K[AlEt₄], Na[AlEt₄] to AlEt₃ isfrom 1:0.5 to 1:3.
 4. The method according to claim 2, wherein 0 to 25mole-% Na[AlEt₄] is employed, relative to the mixture of the complexesK[AlEt₄] and Na[AlEt₄].
 5. The method according to claim 2, wherein amixture of 0.8 mol K[AlEt₄], 0.2 mol Na[AlEt₄], 2.0 mol AlEt₃ in 3.3 moltoluene is used as electrolyte.
 6. The method according to claim 1,wherein a mixture of Na[Et₃Al—H—AlEt₃] and Na[AlEt₄] and AlEt₃ is usedas electrolyte.
 7. The method according to claim 6, wherein a molarratio of Na[Et₃Al—H—AlEt₃] to Na[AlEt₄] is from 4:1 to 1:1.
 8. Themethod according to claim 7, wherein a molar ratio of Na[AlEt₄] to AlEt₃is 1:2.
 9. The method according to claim 8, wherein a mixture of 1 molNa[Et₃Al—H—AlEt₃], 0.5 mol Na[AlEt₄] and 1 mol AlEt₃ in 3 mol toluene isused as electrolyte.
 10. The method according to claim 1, whereinelectrolytic coating is effected performed at temperatures of from 80 to105° C.
 11. The method according to claim 1, wherein pretreatment isperformed for a period of from 1 to 20 minutes.
 12. The method accordingto claim 1, wherein pretreatment is performed at an anodic load of thematerial with a current density of from 0.2 to 2 A/dm².
 13. The methodof claim 3, wherein the molar ratio of said complexes K[AlEt₄],Na[AlEt₄] to AlEt₃ is 1:2.
 14. The method according to claim 4 wherein 5to 20 mole-% Na[AlEt₄] is employed, relative to the mixture of thecomplexes K[AlEt₄] and Na[AlEt₄].
 15. The method of claim 7, wherein themolar ratio of Na[Et₃Al—H—AlEt₃] to Na[AlEt₄] is 2:1.
 16. The method ofclaim 10, wherein electrolytic coating is performed at temperatures offrom 91 to 100° C.
 17. The method of claim 11, wherein pretreatment isperformed for a period of from 5 to 15 minutes.
 18. The method of claim12, wherein pretreatment is performed at an anodic load of the materialwith a current density of from 0.5 to 1.5 A/dm².